N-doped graphene encapsulated FeNi core–shell with S defects for the oxygen evolution reaction

Abstract

The synergistic effect between the transition metal sulfide alloy core, the N-doped graphene shell, and the internal interfacial potential serves to regulate the electronic structure and facilitate electron transfer. We used Fe-based Prussian blue analogues as a single precursor to prepare a N-doped graphene-coated metal core–shell structure (FeNi@NG) through hydrothermal and high-temperature solid phase reactions. The S source was then introduced and annealed, yielding a S, N co-doped graphene-encapsulated metal core–shell structure (FeNi–S@NG). Finally, a nitrogen-doped graphene-coated metal core–shell structure with S defects (S–FeNi@NG) was obtained through NaBH₄ liquid-phase reduction. The synergistic effect between the transition metal sulfide alloy core and the N-doped graphene shell, in conjunction with the internal interfacial potential regulate the electronic structure and facilitate electron transfer. Numerous sulfur vacancies accelerate electron transfer and enhance OER catalytic performance. The optimized material, S–FeNi@NG, exhibited excellent OER catalytic performance, with an overpotential of 250 mV (at 10 mA cm⁻²) and a Tafel slope of 90.95 mV dec⁻¹ in an alkaline electrolyte.

---

1. Introduction

In recent years, the overuse of fossil fuels has led to energy crises and environmental problems, making the development of environmentally friendly energy sources urgent.^1–3 Hydrogen is considered one of the main options for renewable energy due to its zero emissions and pollution-free nature.^4–7 Water electrolysis using renewable energy is a technology for high-purity hydrogen production.^8–11 The electrocatalytic water splitting process typically involves the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode.^12–18 The oxygen evolution reaction is of great significance for hydrogen production, by electrolytic water splitting using a fuel cell¹⁹ and a metal–air battery, and it is also an important half-reaction.^20,21 However, the four-electron proton-coupled oxygen evolution reaction, which involves the cleavage of the O–H bond and then the formation of an O=O bond during the transfer with the four-electron proton,²² has a high kinetic energy potential that affects the entire reaction rate on the anode^23,24 and requires additional catalysts to accelerate the reaction.^25,26 Currently, noble metal-based Ru/Ir compounds are used as advanced OER electrocatalysts, but their high cost and poor durability limit widespread application,^27,28 and most catalysts have poor electrical conductivity and an insufficient specific surface area.²⁹ Therefore, designing cost-effective and efficient OER electrocatalysts is crucial.^30,31

Transition metals (such as Fe, Co, and Ni) and their derivatives are widely studied as promising substitutes for catalysts due to their abundant natural reserves and high theoretical catalytic activity.^32–35 In nanomaterials, crystal defects effectively adjust the electronic structure and surface properties, and are widely applied.^36,37 Various types of defects exist in materials, including point defects, line defects, plane defects, and bulk phase defects. Point defects can be classified into heteroatom dopants, and vacancies based on their position and composition. Transition metal sulfides are regarded as a new generation of high-performance electrode catalytic materials because of their low cost, open skeleton structure, and unique electrochemical properties. However, low conductivity, rapid activity decline, and significant volume changes of sulfides limit their practical application. Current research on transition metal sulfide (TMC) catalyst electrode materials mainly focuses on morphology control and conductivity modification to improve diffusion kinetics.^38–40 However, few studies have focused on accelerating ion transfer by adjusting a material's internal crystal structure. Constructing sulfur defects on the surface of transition metal electrocatalysts may enhance the electrochemical reaction. The introduction of defects redistributes charge, alters O₂ chemisorption, weakens the O–O bond, and improves the catalyst's performance.⁴¹

Here, we prepared N-doped graphene encapsulated FeNi core–shell structure materials (FeNi@NG) using Fe-based Prussian blue analogues (PBA) as a single precursor via hydrothermal and high-temperature solid-state reactions. The S-source was then introduced and reduced by NaBH₄ to form a nitrogen-doped graphene-encapsulated FeNi core–shell structure catalyst (S–FeNi@NG) with sulfur defects. The results indicate that introducing appropriate defects enhances the oxygen evolution performance of the electrocatalyst. Due to its unique structure and composition, the resulting S–FeNi@NG composite exhibited excellent OER performance in 1.0 M KOH solution. The overpotential was approximately 250 mV at a current density of 10 mA cm⁻², with a Tafel slope of 90.95 mV dec⁻¹. The excellent OER activity can be attributed to the following aspects: (1) the synergistic effect between the transition metal sulfide alloy core, the N-doped graphene shell, and internal interfacial potential regulates the electronic structure and promotes electron transfer; (2) numerous sulfur vacancies accelerate electron transfer and enhance OER catalytic performance; and (3) the synthesized catalyst exhibits a large number of active surface sites, improving the electronic structure, electron transfer capacity, and charge density. This research offers a new approach for controlling the surface properties to regulate catalytic behavior by leveraging interface defects. It also provides guidance for the rational design of engineered heterogeneous structures.

2. Experimental section

2.1 Materials

K₃FeC₆N₆ (≥99.5%, AR, Aladdin), Ni (NO₃)₂·6H₂O (≥99.0%, AR, Aladdin), CH₄N₂S (≥99.0%, AR, Aladdin), NaBH₄ (≥98%, Sinopharm Chemical Reagent Co., Ltd), RuO₂ (≥99.9%, Shanghai Hesen Electric Co., Ltd), Ketjen black (Shanghai Hesen Electric Co., Ltd), and 5 wt% Nafion (Sigma-Aldrich) were used.

2.2 Synthesis of FeNi-based core–shell nanostructures

Prussian blue analogues (FeNi-PBA) were synthesized via a simple hydrothermal method. In the typical synthesis process, 10 mL 25 mM K₃[Fe(CN)₆] and 10 mL 25 mM Ni(NO₃)₂·6H₂O solutions were transferred to a 40 mL glass bottle, and the pH was adjusted to 1.1 using concentrated hydrochloric acid. The solution was then thoroughly mixed by magnetic stirring and heated in an 80 °C oven for 20 hours. After the reaction, the supernatant was removed, and the solid samples were collected by centrifugation, repeatedly washed with distilled water and ethanol, and dried at 60 °C overnight. The dried Prussian blue analogues (FeNi-PBA) were annealed at 600 °C at a rate of 10 °C min⁻¹ in an Ar atmosphere for 1 hour to disrupt the precursor's skeleton, and the annealed sample was named FeNi@NG.

2.3 Synthesis of S-doped FeNi-based core–shell nanostructures

The prepared FeNi@NG and CH₄N₂S were placed in a tubular furnace at a mass ratio of 1 : 5, with CH₄N₂S positioned upstream. The mixture was then heated to 400 °C at a rate of 5 °C min⁻¹, maintained in an Ar atmosphere for 2 hours, and cooled to room temperature to obtain the corresponding sulfide product (FeNi–S@NG).

2.4 Synthesis of FeNi-based core–shell nanostructures with S-defects

20 mg of prepared FeNi–S@NG was placed in a 2 mL sample bottle, followed by the addition of 1 M NaBH₄ solution. The mixture was left to stand for 3 hours, after which the upper solution was removed, and the residue was dried in a 60 °C oven. This process yielded a nitrogen-doped graphene encapsulated FeNi core–shell structure catalyst with sulfur defects, named S–FeNi@NG. The entire process is illustrated in Scheme 1 below.

Scheme 1 The synthetic route of the nitrogen-doped graphene-encapsulated FeNi core–shell structure catalyst with S defects.

2.5 Characterization

The microstructure and composition of the samples were analyzed using scanning electron microscopy (SEM, Zeiss Gemini 300), transmission electron microscopy (TEM, Talos F200X), X-ray diffraction (XRD, SmartLab 9 KW), and X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). The Brunauer–Emmett–Teller (BET) equation was applied using the JW-BK200 precision instrument, and the specific surface area was calculated from the nitrogen adsorption–desorption isotherm.

2.6 Preparation of electrodes

Typically, 1 mg of the catalyst and 0.5 mg of Ketjen Black were dispersed in a mixed solvent containing 20 μL of 5 wt% Nafion solution and 250 μL of ethanol to form a homogeneous ink with the aid of sonication for at least 45 minutes. Next, 10 μL of the ink was dropped onto the surface of a polished glassy carbon electrode (GCE, 5 mm diameter) and allowed to dry naturally at room temperature. The mass loading of the active materials was approximately 0.189 mg cm⁻². All electrochemical measurements were conducted using an AutoLab electrochemical workstation with a conventional three-electrode cell. The modified GCE, a graphite rod, and a saturated Ag/AgCl electrode were used as a working electrode, a counter electrode, and a reference electrode, respectively. RuO₂ homogeneous inks were prepared using the same method and applied to a glassy carbon electrode as a reference control.

2.7 Electrochemical tests

The cyclic voltammetry (CV) curve was obtained, and the electrochemical parameters were also characterized by carrying out linear scanning voltammetry (LSV) analysis at 5 mV s⁻¹. All plots displayed were calibrated to a reversible hydrogen electrode (RHE) based on the equation (E_RHE = E_Ag/AgCl + 0.0592 *pH + 0.197) without iR compensation. The OER overpotential is calculated to be: η (V) = E_RHE − 1.23 V (taking the absolute value). Electrochemical impedance spectra (EIS) were obtained by applying an AC voltage with a 10 mV amplitude in the frequency range from 1 Hz to 10⁵ Hz and recorded at −1.35 V vs. RHE for the OER in 1 M KOH (pH = 14). The electrical double-layer specific capacitances (C_dl) of the materials were measured by conducting cyclic voltammetry (CV) at non-Faradaic overpotentials with scan rates of 20, 40, 60, 80, and 100 mV s⁻¹. The stability was tested by chronopotentiometry.

3. Results and discussion

The presence of binary metal alloys and metal sulfides was confirmed by XRD measurements. The 2θ peaks at 43.604°, 50.794°, 74.677°, and 90.630° correspond to the (111), (200), (220), and (311) crystal planes of the FeNi alloy (JCPDS No. 47-1405).^42,43 After doping with the sulfur source, the 2θ peaks at 15.397°, 25.281°, 29.675°, 31.048° and 35.95° align with the (111), (220),(311), (222) and (400) crystal planes of [Co,Fe,Ni]₉S₈ (JCPDS No. 12-0723). Due to the low content, the peak intensity is relatively weak⁴⁴ (Fig. 1).

Fig. 1 XRD patterns of FeNi@NG, FeNi–S@NG and S–FeNi@NG.

Fig. 2a and b show the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the S–FeNi@NG samples, respectively. The nanospheres have particle sizes of approximately 50–100 nm, with rough surfaces, and are stacked together and evenly distributed. The enlarged TEM image in Fig. 2c shows that the spherical sample consists of small encapsulated alloy particles with a binary alloy core and an N-doped graphene shell, forming a core–shell structure. The HRTEM analysis results in Fig. 2d reveal that the thickness of graphene layers is approximately 0.35 nm. Additionally, the distance between the innermost carbon layer and the surface of the metal alloy core is about 0.38 nm. The distance between graphite layers increases with temperature in a roughly linear manner, although the change is minimal.⁴⁵ Bao's group demonstrated that electrons from the metal core can penetrate the graphene shell, enhancing the catalytic process, while the graphene layer contributes to the catalyst's stability.⁴⁶ Therefore, this alloy core with a graphene shell structure is desirable for electrocatalysts. Fig. 2e and f are the SAED and EDS patterns of the S–FeNi@NG sample, respectively. The high-angle annular dark-field scanning transmission electron microscopy image (Fig. 2g) shows the contrast between the central and outer regions, further confirming that the nanocrystal adopts a core–shell structure. The element mapping (Fig. 2h–l) shows that sulfur is doped at the core of the ternary metal alloy, though in low amounts, while nitrogen is doped on the surface of the graphene shell.

Fig. 2 (a)–(l) Characterization analysis of S–FeNi@NG: (a) SEM image (b) and (c) TEM images, (d) HRTEM image, (e) SAED pattern, (f) EDS composition, and (g)–gl) STEM images and elemental maps.

Additionally, we used the Brunauer–Emmett–Teller (BET) method to assess the specific surface area and pore size distribution of S–FeNi@NG. As shown in Fig. 3a, S–FeNi@NG exhibits a typical type IV isotherm in the P/P₀ range of 0.4–1.0. In the higher P/P₀ region, capillary condensation occurs, causing the isotherms to increase rapidly. When all the pores coalesced, adsorption occurs only on the outer surface, which has a much smaller area than the inner surface, resulting in a flattened curve. The hysteresis loop is evident, primarily due to the presence of a mesoporous structure. The pore size distribution is shown in Fig. 3b. The specific surface area of S–FeNi@NG is 45.169 m² g⁻¹, with a mean pore diameter of 15.811 nm and a total pore volume of 0.118 cm³ g⁻¹.

Fig. 3 (a) N₂ adsorption–desorption isotherms of S–FeNi@NG; (b) the pore size distribution of S–FeNi@NG.

The elemental species and valence states of the sample surface were further characterized using X-ray photoelectron spectroscopy (XPS). The survey spectrum in Fig. 4a identifies the main components as Fe, Ni, C, N, and O, with O primarily resulting from inevitable surface oxidation in air. The C spectrum in Fig. 4b shows that carbon exists in C=C (284.5 eV), C–N (285.8 eV), and C=O (288.6 eV) bonds. The N 1s spectrum (Fig. 4c) can be divided into three components corresponding to overlapping pyridine-N (398.5 eV), pyrrole-N (400.3 eV) and graphite-N (403.4 eV). Both pyridine-N and graphite-N contribute to electrocatalytic activity.^47,48 In the Ni and Fe spectra (Fig. 4d and e), the lowest binding energy peaks correspond to metallic Ni (852.7 eV) and Fe (707.2 eV) from FeNi alloys.⁴⁹ Peaks at 855.4 eV and 873.25 eV correspond to Ni³⁺ (2p_3/2 and 2p_1/2), with satellite peaks at 862.9 eV and 880.1 eV.⁵⁰ Similarly, peaks at 710.1 eV and 724.6 eV correspond to Fe (2p_3/2 and 2p_1/2), with a satellite peak at 715 eV.⁵¹ In Fig. 4f, the S 2p spectrum consists of a S 2p_3/2 peak at 162.46 eV and a S 2p_1/2 peak at 161.16 eV, consistent with those of divalent sulfide (S²⁻).^52,53 The higher binding energy of sulfur species of 169.14 eV is attributed to oxidation, indicating sulfur-associated surface reconstruction.⁵⁴ Additionally, the metal content measured by XPS is approximately equal (Table S1, ESI†), consistent with the synthesis ratio.

Fig. 4 XPS spectra of S–FeNi@NG. (a) Survey scan. (b) C 1s, (c) N 1s, (d) Fe 2p, (e) Ni 2p and (f) S 2p peaks of S–FeNi@NG.

The OER activity of the samples was tested in a 1 M KOH solution using a typical three-electrode system. As shown in Fig. 5a, a linear sweep voltammogram (LSV) in 1 M KOH solution was used to assess the oxygen evolution reaction (OER) activity of the sample. Additionally, the performance of a commercial RuO₂ electrocatalyst was tested as a reference. The S–FeNi@NG catalyst exhibits a current density of 10 mA cm⁻² at an overpotential of just 250 mV, significantly lower than those of FeNi@NG (325 mV), FeNi–S@NG (300 mV), and RuO₂ (390 mV), as shown in Fig. 5b. The linear region of the Tafel plot was fitted to the Tafel equation (η = b log j + a, where b is the Tafel slope and j is the current density) to determine the catalytic dynamics of the OER. As shown in Fig. 5c, the Tafel slope was 90.95 mV dec⁻¹ for S–FeNi@NG, 89.84 mV dec⁻¹ for FeNi@NG, 65.23 mV dec⁻¹ for FeNi–S@NG, and 145.11 mV dec⁻¹ for RuO₂. These results indicate that S–FeNi@NG exhibits excellent OER catalytic activity. The Nyquist plots in Fig. 5d reveal that S–FeNi@NG has smaller semicircle diameters compared to FeNi@NG, FeNi–S@NG, and RuO₂, indicating lower charge-transfer resistance at the catalyst–electrolyte interface. Charge-transfer resistance is commonly correlated with electrocatalytic kinetics; smaller charge-transfer resistance indicates better conductivity of the electrode material.

Fig. 5 (a) LSV curve of RuO₂, FeNi@NG, FeNi–S@NG and S–FeNi@NG at 5 mV s⁻¹ in 1.0 M KOH solution without iR-compensation; (b) overpotential; (c) corresponding Tafel plots; and (d) EIS Nyquist plots.

Additionally, the electrochemical double-layer capacitance (C_dl) was determined by cyclic voltammetry (CV) and assumed to be linearly proportional to the electrochemically active surface area (ECSA). As shown in Fig. 6a–c, CV curves were collected from FeNi@NG, FeNi–S@NG, and S–FeNi@NG at different scan rates (20, 40, 60, 80 and 100 mV s⁻¹) in the non-Faraday region of 1 M KOH solution. The C_dl of S–FeNi@NG is 28.03 mF cm⁻², higher than those of FeNi@NG (13.38 mF cm⁻²) and FeNi–S@NG (15.25 mF cm⁻²), as shown in Fig. 6d. The high ECSA value further demonstrates its favorable reaction kinetics, as previous studies have shown that a large ECSA facilitates the transport of electrolytes to catalytically active surfaces.

Fig. 6 (a)–(c) CV test at different scanning rates in 1 M KOH (20, 40, 60, 80, and 100 mV s⁻¹) in the voltage range of 0.13–0.23 V, and calculated C_dl of the sample, and (d) the CV fitting curve of the corresponding catalyst in the non-faraday region.

Long-term stability was also tested by chronopotentiometry at a current density of 10 mA cm⁻². As shown in Fig. 7a, the current density of S–FeNi@NG decreased slightly during the first 2 hours but remained nearly constant for the subsequent 10 hours. The LSV curve in Fig. 7b shows a voltage shift of only 5 mV before and after testing, further confirming the sample's stability.

Fig. 7 (a) Long-term stability test of the S–FeNi@NG electrocatalyst at 10 mA cm⁻², and (b) LSV curves before and after the stability test.

4. Conclusion

In conclusion, this study developed a strategy for synthesizing a high-efficiency electrocatalytic oxygen evolution reaction (OER) electrocatalyst. The key to our success was the growth of the FeNi alloy in a confined space using a metal–organic framework precursor, followed by heteroatomic doping to obtain an N-doped graphene-coated transition metal core–shell structure with sulfur defects, achieved through NaBH₄ liquid-phase reduction. The catalyst exhibited excellent OER activity, achieving a current density of 10 mA cm⁻² at an overpotential of 250 mV in 1.0 M KOH. The excellent OER activity is primarily attributed to the synergistic effect of metal and non-metallic elements, with the internal interfacial potential promoting electron transfer and regulating the electronic structure. This method offers a strategy to control the surface properties of materials, thus regulating their catalytic performance, and may open up new avenues for the rational design and synthesis of advanced materials for electrochemical applications.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the NSFC (62374107), National Key Research and Development Program of China (2022YFC3104700), Key scientific research project of Anhui Colleges and Universities (2023AH052220), Suzhou University PhD Research Start-up Fund Project (2022BSK018), Class III Peak Discipline of Shanghai-Materials Science and Engineering (High-Energy Beam Intelligent Processing and Green Manufacturing) and Shanghai Local University Capacity Building Project of Science and Technology Innovation Action Program (21010501700).

Notes and references

1. J. Masud, P. C. Ioannou, N. Levesanos, P. Kyritsis and M. Nath, A Molecular Ni-complex containing tetrahedral nickel selenide core as highly efficient electrocatalyst for water oxidation, ChemSusChem, 2016, 9(22), 3128–3132.
2. S. Anantharaj, S. Chatterjee, K. C. Swaathini, T. S. Amarnath, E. Subhashini, D. K. Pattanayak and S. Kundu, Stainless steel scrubber: a cost efficient catalytic electrode for full water splitting in alkaline medium, ACS Sustainable Chem. Eng., 2018, 6(2), 2498–2509.
3. M. Zhao, S. Zhang, J. Lin, W. Hu and C. M. Li, Synergic effect of Fe-doping and Ni₃S₂/MnS heterointerface to boost efficient oxygen evolution reaction, Electrochim. Acta, 2022, 430, 141088.
4. H. Wang, Y. Wang, L. Tan, L. Fang, X. Yang, Z. Huang and Y. Wang, Co-Ni based hybrid transition metal oxide nanostructures for cost-effective bi-functional electrocatalytic oxygen and hydrogen evolution reactions, Appl. Catal., B, 2019, 244, 568–575.
5. G. M. Kumar, P. Ilanchezhiyan, C. Siva, A. Madhankumar, T. W. Kang and D. Y. Kim, Ultrafast fabrication of porous transition metal foams for efficient electrocatalytic water splitting, Int. J. Hydrogen Energy, 2020, 45(1), 391–400.
6. J. Zhou, L. Yu, Q. Zhou, C. Huang, Y. Zhang, B. Yu and Y. Yu, A general approach to the synthesis of transition metal phosphide nanoarrays on MXene nanosheets for pH-universal hydrogen evolution and alkaline overall water splitting, Appl. Catal., B, 2021, 288, 120002.
7. E. A. Kamar, K. F. Qasim and M. A. Mousa, Supercapacitor and oxygen evolution reaction performances based on rGO and Mn₂V₂O₇ nanomaterials, Electrochim. Acta, 2022, 430, 141106.
8. L. Yan, B. Zhang, S. Wu and J. Yu, A general approach to the synthesis of transition metal phosphide nanoarrays on MXene nanosheets for pH-universal hydrogen evolution and alkaline overall water splitting, J. Mater. Chem. A, 2020, 8(28), 14234–14242.
9. S. Li, E. Li, X. An, X. Hao, Z. Jiang and G. Guan, Transition-metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives, Nanoscale, 2021, 13, 12788–12817.
10. B. W. Xue, C. H. Zhang, Y. Z. Wang, W. W. Xie, N. W. Li and L. Yu, Recent progress of Ni-Fe layered double hydroxide and beyond towards electrochemical water splitting, Nanoscale Adv., 2020, 2(12), 5555–5566.
11. E. Wang, B. Zhang, J. Zhou and Z. Sun, High catalytic activity of MBenes-supported single atom catalysts for oxygen reduction and oxygen evolution reaction, Appl. Surf. Sci., 2022, 604, 154522.
12. I. Concina, Z. H. Ibupoto and A. Vomiero, Semiconducting metal oxide nanostructures for water splitting and photovoltaics, Adv. Energy Mater., 2017, 7(23), 1700706.
13. N. T. Suen, S. F. Hung, Q. Quan, N. Zhang, Y. J. Xu and H. M. Chen, Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives, Chem. Soc. Rev., 2017, 46(2), 337–365.
14. M. S. Burke, L. J. Enman, A. S. Batchellor, S. Zou and S. W. Boettcher, Oxygen evolution reaction electrocatalysis on transition metal oxides and (oxy) hydroxides: activity trends and design principles, Chem. Mater., 2015, 27(22), 7549–7558.
15. R. Atchudan, T. N. J. I. Edison, S. Perumal, M. Shanmugam and Y. R. Lee, Facile one-pot synthesis of novel structured IONP@C-HIOP composite as superior electrocatalyst for hydrogen evolution reaction and aqueous waste investigation of bio-imaging applications, J. Mol. Liq., 2018, 268, 343–353.
16. L. Zhang, Y. Guo, A. Iqbal, B. Li, D. Gong, W. Liu and W. Qin, One-step synthesis of the 3D flower-like heterostructure MoS₂/CuS nanohybrid for electrocatalytic hydrogen evolution, Int. J. Hydrogen Energy, 2018, 43(3), 1251–1260.
17. L. Chen, L. Hu, C. Xu, L. Yang, W. Ren, W. Wang and Z. Hou, Flexible self-supporting metal-free N-doped graphene membrane as an electrocatalyst for oxygen evolution reaction, Appl. Surf. Sci., 2022, 604, 154667.
18. K. Ao, Q. F. We.i and W. A. Daoud, MOF-Derived Sulfide-Based Electrocatalyst and Scaffold for Boosted Hydrogen Production, ACS Appl. Mater. Interfaces, 2020, 12, 33595–33602.
19. N. L. W. Septiani, Y. V. Kaneti, K. B. Fathoni, Y. N. Guo, Y. Lde, B. Yuliarto, X. C. Jiang, Nugraha, H. K. Dipojono, D. Golberg and Y. Yamauchi, Tailorable nanoarchitecturing of bimetallic nickel cobalt hydrogen phosphate via the self-weaving of nanotubes for efficient oxygen evolution, J. Mater. Chem. A, 2020, 8(6), 3035–3047.
20. J. Song, C. Wei, Z. F. Huang, C. Liu, L. Zeng, X. Wang and Z. J. Xu, A review on fundamentals for designing oxygen evolution electrocatalysts, Chem. Soc. Rev., 2020, 49(7), 2196–2214.
21. H. Pan, X. Zhang, Y. Jia, Y. Zhang, Z. Jiang, C. Sun and K. Wang, Interfacial composite FeOOH enhanced efficient electrocatalytic oxygen evolution of NiSe₂/Ni₂O₃, J. Alloys Compd., 2022, 926, 166779.
22. P. Bhanja, Y. Kim, B. Paul, J. J. Lin, S. M. Alshehri, T. Ahamad, Y. V. Kaneti, A. Bhaumik and Y. Yamauchi, Facile Synthesis of Nanoporous Transition Metal-Based Phosphates for Oxygen Evolution Reaction, ChemCatChem, 2020, 12(7), 2091–2096.
23. X. Chen, Y. Meng, T. Gao, J. Zhang, X. Li, H. Yuan and D. Xiao, An iron foam acts as a substrate and iron source for the in situ construction of a robust transition metal phytate electrocatalyst for overall water splitting, Sustainable Energy Fuels, 2020, 4(1), 331–336.
24. H. S. Gugtapeh and M. Rezaei, Facile electrochemical synthesis of Ni-Sb nanostructure supported on graphite as an affordable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions, J. Electroanal. Chem., 2022, 922, 116726.
25. S. N. Hu, S. Q. Wang, C. Q. Feng, H. M. Wu, J. Zhang and H. Mei, Novel MOF-Derived Nickel Nitrideas High-Performance Bifunctional Electrocatalysts for Hydrogen Evolution and Urea Oxidation, ACS Sustainable Chem. Eng., 2020, 8, 7414–7422.
26. P. Bhanja, Y. Kim, B. Paul, Y. V. Kaneti, A. Alothman, A. Bhaumik and Y. Yamauchi, Microporous nickel phosphonate derived heteroatom doped nickel oxide and nickel phosphide: Efficient electrocatalysts for oxygen evolution reaction, Chem. Eng. J., 2021, 405, 126803.
27. J. Yu, Q. He, G. Yang, W. Zhou, Z. Shao and M. Ni, Recent advances and prospective in ruthenium-based materials for electrochemical water splitting, ACS Catal., 2019, 9(11), 9973–10011.
28. J. Li, Q. He, Y. Lin, L. Han and K. Tao, MOF-Derived Iron–Cobalt Bimetallic Selenides for Water Electrolysis with High-Efficiency Oxygen Evolution Reaction, Inorg. Chem., 2022, 61(47), 19031–19038.
29. Y. Guo, C. R. Zhang, J. H. Zhang, K. Dastafkan, K. Wang, C. Zhao and Z. Q. Shi, Metal−Organic Framework-Derived Bimetallic NiFe Selenide Electrocatalysts with Multiple Phases for Efficient Oxygen Evolution Reaction, ACS Sustainable Chem. Eng., 2021, 9, 2047–2056.
30. N. T. Suen, S. F. Hung, Q. Quan, N. Zhang, Y. J. Xu and H. M. Chen, Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives, Chem. Soc. Rev., 2017, 46(2), 337–365.
31. Z. Xie, Z. Song, J. Zhao, Y. Li, X. Cai, D. Liu and P. Tsiakaras, CuZr Metal Glass Powder as Electrocatalysts for Hydrogen and Oxygen Evolution Reactions, Catalysts, 2022, 12(11), 1378.
32. J. Zhao, J. J. Zhang, Z. Y. Li and X. H. Bu, Recent Progress on NiFe-Based Electrocatalysts for the Oxygen Evolution Reaction, Small, 2020, 16(51), 2003916.
33. M. Tahir, L. Pan, R. Zhang, Y. C. Wang, G. Shen, I. Aslam and J. J. Zou, High-valence-state NiO/Co₃O₄ nanoparticles on nitrogen-doped carbon for oxygen evolution at low overpotential, ACS Energy Lett., 2017, 2(9), 2177–2182.
34. A. QayoomMugheri, U. Aftab, M. IshaqAbro, S. R. Chaudhry, L. Amaral and Z. H. Ibupoto, Co₃O₄/NiO bifunctional electrocatalyst for water splitting, Electrochim. Acta, 2019, 306, 9–17.
35. N. Karthik, T. N. J. I. Edison, R. Atchudan, D. Xiong and Y. R. Lee, Electro-synthesis of sulfur doped nickel cobalt layered double hydroxide for electrocatalytic hydrogen evolution reaction and supercapacitor applications, J. Electroanal. Chem., 2019, 833, 105–112.
36. D. Xue, H. Xia, W. Yan, J. Zhang and S. Mu, Defect engineering on carbon-based catalysts for electrocatalytic CO₂ reduction, Nano-Micro Lett., 2021, 13(1), 1–23.
37. G. Yilmaz, C. F. Tan, Y. F. Lim and G. W. Ho, Pseudomorphic transformation of interpenetrated prussian blue analogs into defective nickel iron selenides for enhanced electrochemical and photo-electrochemical water splitting, Adv. Energy Mater., 2019, 9(1), 1802983.
38. Q. Fu, J. Han, X. Wang, P. Xu, T. Yao, J. Zhong and B. Song, 2D transition metal dichalcogenides: Design, modulation, and challenges in electrocatalysis, Adv. Mater., 2021, 33(6), 1907818.
39. G. Fu and J. M. Lee, Ternary metal sulfides for electrocatalytic energy conversion, J. Mater. Chem. A, 2019, 7(16), 9386–9405.
40. C. Tan, X. Cao, X. J. Wu, Q. He, J. Yang, X. Zhang and H. Zhang, Recent advances in ultrathin two-dimensional nanomaterials, Chem. Rev., 2017, 117(9), 6225–6331.
41. C. Zhang, Y. Shi, Y. Yu, Y. Du and B. Zhang, Engineering sulfur defects, atomic thickness, and porous structures into cobalt sulfide nanosheets for efficient electrocatalytic alkaline hydrogen evolution, ACS Catal., 2018, 8(9), 8077–8083.
42. M. Wu, B. Guo, A. Nie and R. Liu, Tailored architectures of FeNi alloy embedded in N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable zinc-air battery, J. Colloid Interface Sci., 2020, 561, 585–592.
43. G. L. Li, B. B. Yang, X. C. Xu, S. Cao, Y. Shi, Y. Yan and C. Hao, FeNi Alloy Nanoparticles Encapsulated in Carbon Shells Supported on N-Doped Graphene-Like Carbon as Efficient and Stable Bifunctional Oxygen Electrocatalysts, Chem. – Eur. J., 2020, 26(13), 2890–2896.
44. G. Janani, S. Yuvaraj, S. Surendran, Y. Chae, Y. Sim, S. J. Song and U. K. Sim, Enhanced bifunctional electrocatalytic activity of Ni-Co bimetallic chalcogenides for efficient water-splitting application, J. Alloys Compd., 2020, 846, 156389.
45. P. L. Walker, H. A. McKinstry and C. C. Wright, X-Ray Diffraction Studies of a Graphitized Carbon-Changes in Interlayer Spacing and Binding Energy with Temperature, Ind. Eng. Chem., 1953, 45(8), 1711–1715.
46. L. Yu, D. Deng and X. Bao, Chain mail for catalysts, Angew. Chem., Int. Ed., 2020, 132(36), 15406–15409.
47. Q. Zhao, C. Wang, H. Wang and J. Wang, An ultra-dispersive, nonprecious metal MOF-FeZn catalyst with good oxygen reduction activity and favorable stability in acid, J. Mater. Sci., 2021, 56(14), 8600–8612.
48. S. Wu, X. Lv, D. Ping, G. Zhang, S. Wang, H. Wang and S. Fang, Highly exposed atomic Fe-N active sites within carbon nanorods towards electrocatalytic reduction of CO₂ to CO, Electrochim. Acta, 2020, 340, 135930.
49. C. Xuan, J. Wang, W. Xia, Z. Peng, Z. Wu, W. Lei and D. Wang, Porous structured Ni-Fe-P nanocubes derived from a prussian blue analogue as an electrocatalyst for efficient overall water splitting, ACS Appl. Mater. Interfaces, 2017, 9(31), 26134–26142.
50. N. Song, S. Hong, M. Xiao, Y. Zuo, E. Jiang, C. Li and H. Dong, Fabrication of Co(Ni)-P surface bonding states on core-shell Co(OH)₂@ P-NiCo-LDH towards electrocatalytic hydrogen evolution reaction, J. Colloid Interface Sci., 2021, 582, 535–542.
51. J. Y. Xie, Z. Z. Liu, J. Li, L. Feng, M. Yang, Y. Ma and B. Dong, Fe-doped CoP core–shell structure with open cages as efficient electrocatalyst for oxygen evolution, J. Energy Chem., 2020, 48, 328–333.
52. S. Niu, W. J. Jiang, T. Tang, L. P. Yuan, H. Luo and J. S. Hu, Autogenous growth of hierarchical NiFe(OH)x/FeS nanosheet-on-microsheet arrays for synergistically enhanced high-output water oxidation, Adv. Funct. Mater., 2019, 29(36), 1902180.
53. L. Hu, Z. Wei, F. Yu, H. Yuan, M. Liu, G. Wang and J. Ma, Defective ZnS nanoparticles anchored in situ on N-doped carbon as a superior oxygen reduction reaction catalyst, J. Energy Chem., 2019, 39, 152–159.
54. M. Yao, H. Hu, N. Wang, W. Hu and S. Komarneni, Quaternary (Fe/Ni) (P/S) mesoporous nanorods templated on stainless steel mesh lead to stable oxygen evolution reaction for over two months, J. Colloid Interface Sci., 2020, 561, 576–584.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04244a

---